OSLO, Aug 15 (Reuters) - Downpours and heatwaves caused by
climate change could disrupt food supplies from the fields to
the supermarkets, raising the risk of more price spikes such as
this year's leap triggered by drought in the United States.

Food security experts working on a chapter in a U.N.
overview of global warming due in 2014 said governments should
take more account of how extremes of heat, droughts or floods
could affect food supplies from seeds to consumers' plates.

"It has not been properly recognised yet that we are dealing
with a food system here. There is a whole chain that is also
going to be affected by climate change," Professor Dr John
Porter of the University of Copenhagen said.

"It is more than just the fact that there are droughts in
the United States that will reduce yields," he said. Like the
other experts, he said was giving personal opinions, not those
of the U.N. panel.

After harvest, floods could wash away roads or bridges, for
instance, between fields and factories processing the crop. Or
warehouses storing food could be damaged by more powerful
storms. Such factors were likely to hit poor nations hardest.

"There are reasons to expect more frequent (price) spikes,
given that it will be more common to see conditions that are
considered extreme," said David Lobell, an assistant professor
at Stanford University in California.

GRAIN STORAGE

Other factors could dampen rises, however, "including
responses such as raising grain storage or changing trade
policies". He said Stanford was trying to produce models of the
likelihood of price spikes to understand the risks.

"It's a distributional problem - there is enough food in the
world. But the distribution doesn't work," said Bruce McCarl, a
professor at Texas A&M University. Climate extremes could
aggravate food price swings, he said.

The worst drought in five decades in the United States has
pushed up corn prices by more than 50 percent from late May to
record highs above $8 a bushel. Hot, dry weather has also
hit crops in southern Europe.

A U.N. report on climate extremes in March said it was
"virtually certain" that days with extreme heat would increase.
Among other findings, it said it was likely that downpours would
increase as a percentage of total rainfall.

Scientists are traditionally wary of linking individual
extremes such as the U.S. drought to climate change - weather
events from heatwaves to dust storms have happened throughout
history.

But James Hansen, director of the NASA Goddard Institute for
Space Studies, expressed "a high degree of confidence" this
month that the European heatwave of 2003, the Russian heatwave
of 2010 and the Texas and Oklahoma droughts of 2011 were "a
consequence of climate change" because they were so extreme.

His conclusion was challenged as too definite, even by some
experts who say risks of such events are rising with greenhouse
gas emissions, led by China and the United States.

On the positive side for food output, a slight rise in
temperatures is likely to help plant growth overall.

RISING TEMPERATURES

But long-term net benefits are doubtful, especially because
U.N. studies say rising greenhouse gas emissions are on track to
push temperatures up by more than 2 Celsius (3.6 Fahrenheit)
above pre-industrial times, set by 200 nations as a threshold
for dangerous change.

Temperatures have already risen by almost 1 degree (1.6F).
Nations such as Australia could lose out more than others
benefit.

"In Australia there are huge areas where you can grow wheat.
If that goes, I don't think there are northern areas that can
take up the huge production lost," said Kaija Hakala, of MTT
Agrifood Research Finland.

With more frequent climate extremes, researchers said there
would be hard choices with a projected rise in the world
population to 9 billion people by 2050 from 7 billion now. They
urged more research into drought- or flood-resistant crops.

"We may be hitting a point where it's getting harder to get
technological progress" in raising yields, McCarl said. Annual
yield growth for U.S. corn had slowed to about 1.5 percent from
stellar rates of about 3.5 percent in the early 1970s.

Porter said the world had so far escaped predictions that
population growth would outstrip food production, most famously
by English writer Thomas Malthus in 1798.

But he said the world now had triple goals of producing food
for people, crops for biofuels and feed for animals, often
raised for their meat. "In my view we can have two out of those
three and not all three," he said.